2 full days are barely enough for Florence. The best way to enjoy the countryside would be with a rental car. For example a drive to Siena through the Via Chiantigiana (Regional Road 222) would be a very popular option. However, whatever you choose to do, will likely require the best part of a full day.

Lindsay, If you're already committed to "1.5 organized days in Florence and a day trip to a hilltown in Tuscany", I'm not sure you'll have time in 2 full days to do much more. Any time you're able to spend in the countryside will be very short, so you won't be able to linger too long. Good luck and happy travels!

Looks like you might be pushed for time, however if you have few hours free in the evening you could take a cooking class with Cook Eat Italian- http://www.cookeatitalian.com/ - instead of having a meal out at a restaurant. They do some day classes, evening classes and even a day out in the Tuscan countryside.